1. Field of the Invention
The invention relates to a vehicle steering system that includes a motor control device for driving a brushless motor. One example of the vehicle steering system is an electric power steering system.
2. Description of Related Art
A motor control device for carrying out drive control over a brushless motor is generally configured to control current that is supplied to the motor based on a signal output from a rotation angle sensor for detecting the rotation angle of a rotor. However, when the rotation angle sensor fails, motor control is no longer continued. Then, there has been suggested a sensorless drive method according to which a brushless motor is driven without using a rotation angle sensor. The sensorless drive method is a method for estimating the phase of a magnetic pole (the electrical angle of a rotor) by estimating induced voltage resulting from rotation of the rotor. During stop of the rotor or during extremely low speed rotation, induced voltage cannot be estimated, so the phase of the magnetic pole is estimated according to another method. Specifically, a sensing signal is transmitted to a stator and then a response of the motor to the sensing signal is detected. A rotor rotation position is estimated based on the response of the motor. For example, Japanese Patent Application Publication No. 10-243699 (JP-A-10-243699) and Japanese Patent Application Publication No. 2009-124811 (JP-A-2009-124811) describe related arts.
According to the sensorless drive method, induced voltage and a sensing signal are used to estimate the rotation position of the rotor and then the motor is controlled based on the estimated rotation position. However, this drive method is not suitable for all applications. A method for applying the sensorless drive method to control over a brushless motor that is used as a driving source of a vehicle steering system, such as an electric power steering system that applies steering assist force to a steering mechanism of a vehicle, has not yet established. Therefore, another method for implementing sensorless control is expected.